The present invention relates to an organic electrolytic cell, which has a high capacity and high voltage and is superior in charge and discharge characteristics and safety.
In recent years, a secondary cell wherein an electrically conductive polymer, an oxide of a transition metal or the like is used as the positive electrode, and metallic lithium or a lithium alloy is used as the negative electrode has been proposed as a cell to be used in place of Nixe2x80x94Cd cells and lead-acid cells, because of its high energy density.
However, when such a secondary cell is subjected to repeated charge and discharge, its capacity is largely lowered due to deterioration of the positive or negative electrode, and thus there still remains a problem in its practical aspect. Particularly by deterioration of the negative electrode, mossy lithium, called dendrites, are formed, and through repeated charge and discharge, the dendrites finally pierce the separator and cause short-circuit. In some case, the cell is broken and thus there has been a problem in safety, too.
To solve the above problems, there has been proposed a cell wherein a carbon material such as graphite is used as the negative electrode and a lithium-containing metallic oxide such as LiCoO2 is used as the positive electrode. This cell is a so-called rocking chair-type cell wherein, after assembly of the cell, lithium is supplied from the lithium-containing metallic oxide as the positive electrode to the negative electrode through charge, and lithium of the negative electrode is returned to the positive electrode through discharge. Although the cell is characterized by a high voltage and high capacity, the high energy density as an advantage of the lithium cells has not been obtained.
In recent years, with the rapid progress of a study about a negative electrode material capable of reversibly carrying lithium, a material capable of carrying lithium in the amount exceeding that of C6Li, which is a theoretic amount of the carbon material, and an oxide of tin have been proposed as the negative electrode material for high-capacity lithium secondary cells. Among them, an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.5 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer, is capable of doping with lithium up to C2Li (Synthetic Metals, 73 (1995) P273). However, the above locking chair-type cell wherein this infusible and insoluble substrate is used as the negative electrode and the lithium-containing metallic oxide as the positive electrode can attain a capacity higher than that in the case of the carbon material after assembly, but there still remains an unsatisfactory respect in its capacity.
To solve the above problems, PCT Publication No. WO95/8852, whose application was filed by the present applicant, has proposed an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein the positive electrode contains a metallic oxide, the negative electrode is an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.5 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer, and the total amount of lithium contained in the cell is not less than 500 mAh/g and the amount of lithium contained in the cell is not less than 100 mAh/g, based on the infusible and insoluble substrate as the negative electrode. Although this cell can attain a high capacity, a method of carrying lithium originating in the negative electrode, practically and simply, is required in the case of assembly of a practical cell such as cylindrical-type cell. Various specific methods thereof are proposed in Japanese Patent Kokai (Laid-Open) Publication Nos. 162159/1996, 162160/1996, 162161/1996 and 255633/1996. However, any of these methods has a problem in uniformity and operating property and the problem has still to be completely solved at present. That is, a most simple method in these specific methods includes a method of attaching a lithium metal on a positive or negative electrode, inserting the resultant into a cell container, together with the positive or negative electrode and a separator, pouring an electrolytic solution and allowing to stand, thereby to carry lithium on the positive or negative electrode. However, this method had such a problem that, since a lower limit of the thickness of a lithium metal foil to be attached, which can be mass-produced, is about 30 xcexcm, the thickness of the positive and/or negative electrodes increases thereby to restrict design of the cell and to exert an influence particularly on charge and discharge characteristics.
The present inventors have studied intensively in light of the problems described above, thus completing the present invention. An object of the present invention is to provide an organic electrolytic cell, which is easy to produce, and which has a high capacity and high voltage.
Another object of the present invention is to provide a secondary cell, which is superior in charge and discharge characteristics.
Still another object of the present invention is to provide a secondary cell, which is capable of charging and discharging for a long period and is superior in safety.
A further object of the present invention is to provide a secondary cell having a low internal resistance.
A still further object of the present invention is to provide a secondary cell, which is easy to produce.
Still another objects, features and advantages of the present invention will become apparent from the following description.
To attain these objects, the organic electrolytic cell of the present invention has the following construction. That is, the present invention provides an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein a current collector of the positive electrode and a current collector of the negative electrode are respectively provided with pores piercing from the front surface to the back surface, an active material of negative electrode is capable of reversibly carrying lithium, and lithium originating in the negative electrode is carried on the negative electrode by electrochemical contact with the lithium arranged to face the positive or negative electrode, thereby enabling all or a portion of lithium to permeate into at least one layer of the positive or negative electrode.
It is preferred that a current collector of the positive electrode and a current collector of the negative electrode in the organic electrolytic cell are respectively provided with pores piercing from the front surface to the back surface and a porosity of each current collector is not less than 1% and not more than 30%, an active material of the negative electrode is capable of reversibly carrying lithium, and lithium originating in the negative electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of lithium is not more than 40% of an area of the negative electrode.
The present invention also provides an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein a current collector of the positive electrode and a current collector of the negative electrode are respectively provided with pores piercing from the front surface to the back surface, an active material of the positive electrode and an active material of the negative electrode are capable of reversibly carrying lithium, and lithium originating in the positive electrode is carried on the positive electrode by electrochemical contact with lithium arranged to face the negative or positive electrode, thereby enabling all or a portion of lithium to permeate into at least one layer of the positive or negative electrode.
The active material of the negative electrode is preferably an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.5 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer.
In the organic electrolytic cell according to claim 1 or claim 7, the total amount of lithium contained the cell is not less than 500 mAh/g and the amount of lithium originating in the negative electrode is not less than 100 mAh/g, based on the active material of the negative electrode.
The present invention also provides an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein a current collector of the positive electrode and a current collector of the negative electrode are respectively provided with pores piercing from the front surface to the back surface and a porosity of each current collector is not less than 1% and not more than 30%, an active material of the positive electrode and an active material of the negative electrode are capable of reversibly carrying lithium, and at least one portion of lithium originating in the positive electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of lithium is not more than 40% of an area of the positive electrode.
The active material of the negative electrode is preferably an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.5 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer.
The active material of negative electrode in the organic electrolytic cell of the present invention may be any one capable of reversibly carrying lithium, and examples thereof include graphite, various carbon materials, polyacene substance, tin oxide, silicon oxide and the like. Among them, it is preferred to use an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.5 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer, because a high capacity can be obtained.
The aromatic condensation polymer is a condensate of an aromatic hydrocarbon compound and aldehydes. As the aromatic hydrocarbon compound, for example, so-called phenols such as phenol, cresol, xylenol and the like can be suitably used. There can also be used methylenebisphenols represented by the following formula: 
wherein x and y are independently 0, 1 or 2, or hydroxybiphenyls or hydroxynaphthalenes. For practical purpose, phenols, particularly phenol, are preferred.
As the aromatic condensation polymer, there can also be used a modified aromatic condensation polymer wherein a portion of the aromatic hydrocarbon compound having phenolic hydroxyl groups is replaced with an aromatic hydrocarbon compound having no phenolic hydroxyl group such as xylene, toluene or aniline, for example, a condensate of phenol, xylene and formaldehyde. Furthermore, there can also be used a modified aromatic polymer wherein the above portion is replaced with melamine or urea. A furan resin is also preferred.
As the aldehyde, it is possible to use aldehydes such as formaldehyde, acetaldehyde and furfural, but formaldehyde is preferred. A phenolformaldehyde condensate may be any of a novolak type, a resol type or a mixture thereof.
The infusible and insoluble substrate can be obtained by a heat treatment of the above aromatic polymer, and includes all of infusible and insoluble substrates having a polyacene skeletal structure described in Japanese Patent Publication Nos. 44212/1989 and 24024/1991.
The infusible and insoluble substrate used in the present invention can also be produced as follows. That is, an infusible and insoluble substrate having a hydrogen/carbon atomic ratio (hereinafter referred to as H/C) of 0.5 to 0.05, preferably 0.35 to 0.10 can be obtained by gradually heating the aromatic condensation polymer up to a proper temperature of 400 to 800xc2x0 C. in a non-oxidizing atmosphere (including a vacuum).
It is also possible to obtain an infusible and insoluble substrate having a specific surface area, measured by the BET method, of not less than 600 m2/g according to the method described in Japanese Patent Publication No. 24024/1991. For example, an infusible and insoluble substrate having the above H/C and having a specific surface area, measured by the BET method, of not less than 600 m2/g can also be obtained by preparing a solution containing a initial condensate of an aromatic condensation polymer and an inorganic salt such as zinc chloride; heating the solution to cure it in a mold; gradually heating the cured matter in a non-oxidizing atmosphere (including a vacuum) up to a temperature of 350 to 800xc2x0 C., preferably up to a proper temperature of 400 to 750xc2x0 C.; and then sufficiently washing it with water, diluted hydrochloric acid or the like.
With respect to the infusible and insoluble substrate used in the present invention, according to X-ray diffraction (CuKxcex1), the main peak is observed at 2xcex8=24xc2x0 or below, and besides another broad peak is observed at between 2xcex8=41xc2x0 and 2xcex8=46xc2x0, in addition to the main peak. Namely, it is suggested that the infusible and insoluble substrate has a polyacene skeletal structure wherein an aromatic polycyclic structure is moderately developed, and takes an amorphous structure. Thus the substrate can be doped stably with lithium and, therefore, it is useful as an active material for cells.
It is preferred that this infusible and insoluble substrate has H/C ranging from 0.5 to 0.05. When H/C exceeds 0.5, the aromatic polycyclic structure does not sufficiently develop, and thus it is impossible to conduct doping and undoping of lithium smoothly, and when a cell is assembled, charge and discharge efficiency is lowered. On the other hand, when H/C is less than 0.05, the capacity of the cell of the present invention is likely to be lowered.
The negative electrode in the organic electrolytic cell according to the present invention is composed of the above infusible and insoluble substrate (hereinafter referred to as PAS), and practically, it is preferred to use a form obtained by forming PAS in an easily formable form such as a powdery form, a granular form or a short fiber form with a binder. As the binder, there can be used fluorine-containing resins such as polyethylene tetrafluoride and polyvinylidene fluoride, and thermoplastic resins such as polypropylene and polyethylene. It is preferred to use a fluorine binder. Use of a fluorine binder having a fluorine/carbon atomic ratio (hereinafter referred to as F/C) of less than 1.5 and not less than 0.75 is preferred, and use of a fluorine binder having a fluorine/carbon atomic ratio of less than 1.3 and not less than 0.75 is more preferred.
The fluorine binder includes, for example, polyvinylidene fluoride, vinylidene fluoride-ethylene trifluoride copolymer, ethylene-ethylene tetrafluoride copolymer, propylene-ethylene tetrafluoride or the like. Furthermore, it is also possible to use a fluorine-containing polymer wherein hydrogens at the principal chain are replaced with alkyl groups. In the case of the polyvinylidene fluoride, F/C is 1. In the case of the vinylidene fluoride-ethylene trifluoride copolymer, when the molar fractions of vinylidene fluoride are 50% and 80%, F/C values become 1.25 and 1.1, respectively. In the case of thepropylene-ethylene tetrafluoride copolymer, when the molar fraction of propylene is 50%, F/C becomes 0.75. Among them, polyvinylidene fluoride, and a vinylidene fluoride-ethylene trifluoride copolymer wherein the molar fraction of vinylidene fluoride is not less than 50% are preferred. For practical purpose, polyvinylidene fluoride is preferred.
When using these binders, it is possible to sufficiently utilize the doping ability (capacity) with lithium which PAS has.
When using PAS, oxide or the like as the active material of negative electrode, if necessary, electrically conductive materials such as acetylene black, graphite, metallic powder and the like may be appropriately added in the negative electrode of the organic electrolytic cell of the present invention.
The active material of positive electrode in the organic electrolytic cell according to claim 1 of the present invention is not specifically limited, but there can be used lithium-containing metallic oxides capable of electrochemically doping with lithium and electrochemically undoping lithium, which can be represented by the general formula LixMyOz (M is a metal, or can be two or more metals) such as LixCoO2, LixNiO2, LixMnO2 or LixFeO2, or oxides of transition metals such as cobalt, manganese and nickel. The above electrically conductive polymers such as PAS can also be suitably used. Particularly, when a high voltage and high capacity are required, a lithium-containing oxide having a voltage of not less than 4 V vs lithium metal is preferred. Among them, lithium-containing cobalt oxides, lithium-containing nickel oxides or lithium-containing cobalt-nickel complex oxides are particularly preferred.
The active material of positive electrode in the organic electrolytic cell according to claim 4 of the present invention is not specifically limited, but there can be used lithium-containing metallic oxides (these lithium-containing metal oxides are capable of emitting lithium through electrochemical oxidation, namely charge, and is referred to as a first type of an active material of positive electrode) which can be represented by the general formula LixMyOz (M is a metal, or can be two or more metals) such as LixCoO2, LixNio2, LixMnO2 or LixFeO2, or oxides and sulfides of transition metals such as cobalt, manganese, vanadium, titanium and nickel. The above electrically conductive polymers such as PAS can be suitably used. These active materials of positive electrode can be roughly classified into two kinds. That is, they are an active material of positive electrode (referred to as a first type of an active material of positive electrode in the present invention) capable of emitting lithium through electrochemical oxidation, namely charge, such as lithium-containing nickel oxides, lithium-containing cobalt-nickel double oxides and lithium-containing cobalt-nickel double oxides, and the other active material of positive electrode (referred to as a second type of an active material of positive electrode in the present invention). Particularly, when a high voltage is required, a lithium-containing oxide having a voltage of not less than 4 V vs lithium metal is preferred. Among them, lithium-containing cobalt oxides, lithium-containing nickel oxides or lithium-containing cobalt-nickel complex oxides are particularly preferred.
The positive electrode in the organic electrolytic cell of the present invention is one made by optionally adding an electrically conductive material and a binder to the above each active material and molding the mixture, and the kind and composition of the electrically conductive material and binder can be appropriately specified.
As the electrically conductive material, a powder of a metal such as metallic nickel can be used but carbon material such as active carbon, carbon black, acetylene black and graphite can be suitably used. A mixing ratio of these electrically conductive materials varies depending on the electric conductivity of the active material, shape of the electrode, etc., but it is suitable to add it in an amount of 2 to 40% based on the active material.
The binder may be any one which is insoluble in an electrolytic solution described hereinafter used in the organic electrolytic solution of the present invention. There can be preferably used, for example, rubber binders such as SBR, fluorine-containing resins such as polyethylene tetrafluoride and polyvinylidene fluoride, and thermoplastic resins such as polypropylene and polyethylene. The mixing ratio is preferably not more than 20% based on the above active material.
The current collector of positive electrode and current collector of negative electrode in the organic electrolytic cell of the present invention are respectively provided with pores piercing from the front surface to the back surface, and are made of materials such as nonwoven fabric, expanded metal, punched metal, net, foamed material or the like. The form and number of these through pores are not specifically limited and can be appropriately determined so that lithium ions in the electrolytic solution described hereinafter can transfer between the surface and back surfaces of the electrode without being interrupted by the current corrector of electrode. The porosity of the electrode current corrector is obtained by reducing a ratio of {1-(weight of current corrector)/(true specific gravity of current corrector)/(apparent volume of current corrector)} to percentage.
As the material of the electrode-current corrector, there can be used various materials which are generally proposed in lithium cells. Aluminum and stainless steel can be used as the current corrector of positive electrode, whereas, stainless steel, copper and nickel can be used as the current corrector of negative electrode. With respect to the current corrector of positive electrode, when lithium is directly attached as described hereinafter, it is preferred to use a material, which does not make an alloy with lithium and has resistance to electrochemical oxidation, such as stainless steel.
In the organic electrolytic cell according to claim 1 of the present invention, the total amount of lithium contained the cell is preferably not less than 500 mAh/g and the amount of lithium originating in the negative electrode is preferably not less than 100 mAh/g, based on the active material of negative electrode. The total amount of lithium contained the cell is the total of the amount of lithium originating in the positive electrode, that of lithium originating in the electrolytic solution and that of lithium originating in the negative electrode. Lithium originating in the positive electrode is lithium contained in the positive electrode on assembly of the cell, and a portion or all of lithium is supplied to the negative electrode through an operation of applying a current from an external circuit (charge). Lithium originating in the electrolytic solution in the organic electrolytic cell of the present invention is lithium in the electrolytic solution contained in the separator, positive electrode and negative electrode, whereas, lithium originating in the negative electrode is lithium carried on the active material of negative electrode and is lithium other than lithium originating in the positive electrode and lithium originating in electrolytic solution.
In the organic electrolytic cell according to claim 1 of the present invention, lithium originating in the negative electrode is carried on the negative electrode by electrochemical contact with lithium arranged to face the negative or positive electrode, thereby enabling all or a portion of lithium to permeate into at least one layer of the positive or negative electrode. As used herein, the term xe2x80x9clithiumxe2x80x9d refers to any material, which contains at least lithium and is capable of supplying lithium ions, such as lithium metal, lithium-aluminum alloy or the like.
In the organic electrolytic cell according to claim 1 of the present invention, in case the electrode adjacent to lithium arranged is a negative electrode, lithium is directly carried on the adjacent negative electrode, while lithium, which permeates into at least one layer of the positive electrode, is carried on the other negative electrode which is not adjacent to lithium. In case the electrode adjacent to lithium arranged is a positive electrode, all of lithium is carried on the negative electrode after it permeates into at least one layer of the positive electrode.
In the organic electrolytic cell according to claim 4 of the present invention, lithium originating in the positive electrode is lithium contained in the positive electrode and all or a portion of lithium is carried on the positive electrode by electrochemical contact with lithium arranged to face the negative or positive electrode. For example, when using LiCoO2 as the active material of positive electrode, LiCoO2 has already contained lithium on assembly of the cell, but lithium originating in the positive electrode is obtained by further adding lithium carried through electrochemical contact with lithium. On the other hand, when using V2O5 as the active material of positive electrode, since this material does not contain lithium, all of lithium originating in the positive electrode is carried by electrochemical contact with lithium. A portion or all of this lithium originating in the positive electrode is supplied to the negative electrode through an operation of applying a current from an external circuit (charge). Lithium originating in the electrolytic solution in the organic electrolytic cell of the present invention is lithium in the electrolytic solution contained in the separator, positive electrode and negative electrode, whereas, lithium originating in the negative electrode is lithium carried on the active material of negative electrode and is lithium other than lithium originating in the positive electrode and lithium originating in electrolytic solution.
In the organic electrolytic cell according to claim 4 of the present invention, all or a portion of lithium originating in the positive electrode is carried on the positive electrode by electrochemical contact with lithium arranged to face the negative or positive electrode, thereby enabling lithium to permeate into at least one layer of the positive or negative electrode.
The electrochemical contact between lithium and the positive electrode initiates when the electrolytic solution is poured into the cell system. When using the above first type of an active material of positive electrode, since said active material of positive electrode has already contained releasable lithium, it becomes possible to charge the cell system immediately after pouring the electrolytic solution into the cell system. Also when using the second type of an active material of positive electrode, it is possible to charge the cell system before all lithium is completely carried on the active material of positive electrode after pouring the electrolytic solution into the cell system. The above charge operation is effective to reduce the carrying time and to prevent the positive electrode from being in an over-discharge state, thereby preventing deterioration of the positive electrode due to the carrying operation of lithium.
In the organic electrolytic cell of the present invention, lithium originating in the negative or positive electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode, thereby enabling all or a portion of lithium to permeate into at least one layer of the positive or negative electrode. As used herein, the term xe2x80x9clithiumxe2x80x9d refers to any material, which contains at least lithium and is capable of supplying lithium ions, such as lithium metal, lithium-aluminum alloy or the like.
In the organic electrolytic cell according to claim 1 of the present invention, lithium originating in the negative electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of lithium is not more than 40% of an area of the negative electrode. In the organic electrolytic cell according to claim 4 of the present invention, at least a portion of lithium originating in the negative electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of lithium is not more than 40% of an area of the positive electrode. As used herein, the term xe2x80x9clithiumxe2x80x9d refers to any material, which contains at least lithium and is capable of supplying lithium ions, such as lithium metal, lithium-aluminum alloy or the like.